In marine and industrial construction, it is usually desirable to pre-paint steel with a zinc-rich primer before fabrication, and many such coating compositions are known. Many of these compositions include zinc powder in an organic resin such as an epoxy resin, a chlorinated rubber, a polystyrene resin or a silicone resin. Coatings based on these organic binders are not well suited for coating steel that must ultimately be welded because the binder tends to decompose from the heat of the weld, resulting in damaged coatings and loss of adhesion.
As an alternative to coatings based on organic binders, zinc coatings based on inorganic binders are also known. For example, Schutt (U.S. Pat. No. 3,620,784) discloses a coating composition containing zinc dust carried in a silicate vehicle. The particular formulation disclosed by Schutt is said to avoid many of the problems commonly encountered with inorganic coatings: weather wear, cracking, crazing and non-uniform adherence before and after baking.
However, even if one surmounts the problems associated with the physical characteristics of the applied silicate coating, as Schutt claims to have done, one is still faced with the problem of welding steel that has been coated with zinc dust primer in a silicate vehicle. During welding the vaporized zinc can penetrate the weld root and become entrapped, creating a porous weld. It is sometimes possible to partially overcome the porosity problem by reducing the welding speed sufficiently to allow the zinc to become completely volatized ahead of the weld. Even when this technique is successful, however, it results in increased fabrication costs due to reduced welding speed.
It is known that a portion of zinc in zinc-rich primers can be replaced by iron phosphide of suitable particle size. The resulting zinc-coated steel, while retaining its anti-corrosive properties, can be more readily and efficiently welded.
As an example of this approach, Makishima et al. (U.S. Pat. No. 4,011,088) have proposed an anti-corrosive coating composition comprising 5 to 80% of a binder, which is either potassium silicate or ammonium silicate, and 20 to 95% of a pigment mixture of zinc powder and iron phosphide or zinc powder and nickel phosphide. In the embodiment in which potassium silicate is used as the binder, Makishima et al. indicate that the ratio of silicate to potassium should be between 2.5 and 4.0. If the mole ratio is smaller than 2.5, the film forming property is said to be insufficient and if the mole ratio is larger than 4.0, the stability of the binder is said to be reduced.
I have now discovered an improved zinc based primer that provides a durable, corrosion-resistant coating, combined with excellent weldability. Moreover, the composition is water based and thereby avoids the environmental problems associated with solvent-based primers.